1. Field of the Invention
The present invention relates to lead-containing fluoride glass, an optical fiber with a core made thereof, and a process for fabricating the optical fiber, which may be applicable in the field of infrared sensors, optical amplifiers, or optical fiber lasers.
2. Related Background Art
A fluoride fiber is full of promise as an optical fiber for long distance transmission cables, for infrared sensors, or for optical amplifiers. A typical optical fiber is composed of two parts called a core and a cladding, in which the refractive index of the core, through which light propagates, is higher than that of the cladding. A relative refractive index difference is defined as follows: EQU .DELTA.n=(n.sub.1 -n.sub.2)/n.sub.1 .times.100 (%),
where n.sub.1 is the refractive index of the core and n.sub.2 is the refractive index of the cladding.
A fiber for an infrared sensor, etc., may have a greater input-output of light by increasing a numerical aperture (NA), which is a criterion of range for capturing light. Therefore, a larger relative refractive index difference .DELTA.n is more desirable. A fluoride fiber with a core containing a rare earth element such as Nd (neodymium) or Pr (praseodymium) is also promising as a fiber for optical amplifier in 1.3 .mu.m band of wavelength. In this case, a greater difference .DELTA.n can make an optical power confined in the core and is, therefore, effective to increase a degree of amplification of light.
Incidentally, a most typical composition of the fluoride fiber is the ZBLAN series (Zr--Ba--La--Al--Na--F series). In general, the following methods are known to increase the difference .DELTA.n in a fiber of this type.
(1) A method of making concentrations of Al (aluminum) or Na (sodium) in the core lower than those of Al or Na in the cladding.
(2) A method of adding a heavy metal such as Pb (lead) and Bi (bismuth) into the core.
(3) A method of adding Hf (hafnium) into the cladding.
It is found that the most effective method is the method (2). It is stated for example in Japanese Journal of Applied Physics Vol. 20, No. 5, May, 1981, pp. L 337-L 339 that a high refractive index may be obtained by adding PbF.sub.2 into a glass material basically comprising 33BaF.sub.2 --4GdF--63ZrF.sub.4 (mol %).
The addition of a heavy metal such as Bi into the core can increase the refractive index but makes the core possibly colored, which is unfavorable for the core of an optical fiber in respect of transmission loss. In case that the additive is PbF.sub.2, a fused material is likely to crystallize instead of forming glass. Therefore, the fluoride glass containing Pb has such a problem to be solved that the stable of state is greatly affected depending upon the composition of the entire system.
In connection with this, Japanese Laid-open (Kokai) Patent Application No. 1-321406 discloses that the fluoride glass in a composition of 49ZrF.sub.4 --(25--X)BaF.sub.2 --XPbF.sub.2 --3.5LaF.sub.3 --2YF.sub.3 --2.5AlF.sub.3 --18NaF (mol %) may have a difference .DELTA.n=2.8% by adding PbF.sub.2 up to X=12 (mol %) thereinto.
The present inventors tried to make fluoride glasses with various composition rates X of PbF.sub.2 in accordance with the composition as described in the above Japanese Patent Application. The trial showed that transparent glasses could be obtained with X&lt;5 mol %, but no good glass was obtained with X=5-12 mol % due to crystallization.